With increased development of technologies for mobile devices and requirements for the same, demand for a battery as an energy source of the mobile device has rapidly increased. Under such circumstances, a great deal of study and investigation into novel batteries satisfying various consumer needs has been conducted. Especially, a lithium secondary battery such as a lithium ion battery, a lithium ion polymer battery, etc. having excellent energy density, discharge voltage, output stability and the like are in great demand.
In general, a lithium secondary battery includes a cathode active material based on a metal oxide such as LiCoO2, an anode active material based on a carbon material, a porous polymer membrane interposed between a cathode and an anode, and a non-aqueous electrolyte containing a lithium salt such as LiPF6 provided therein. During charge, lithium ions contained in the cathode active material are released and enter into a carbon layer in the anode, while lithium ions are released from the carbon layer and enter into the cathode active material during discharge. The non-aqueous electrolyte is a medium through which the lithium ions move between the anode and the cathode. Such a lithium secondary battery must be stable in operating voltage ranges of the battery and have a high rate of ion delivery.
If the non-aqueous electrolyte includes only cyclic carbonates with high polarity sufficient to dissociate lithium ions, a viscosity of the electrolyte may be increased, leading to a decrease in ion conductivity thereof.
U.S. Pat. Nos. 5,521,027 and 5,525,443 describe an electrolyte mixture including a linear carbonate having reduced viscosity and polarity as well as a cyclic carbonate in order to decrease a viscosity of the electrolyte. Representative examples of such a linear carbonate include dimethyl carbonate (DMC), diethyl carbonate (DEC), ethylmethyl carbonate (EMC), etc. and, among these compounds, EMC having the lowest freezing point of −55° C. is preferably used to exhibit superior low-temperature performance and lifespan. Representative examples of the cyclic carbonate include ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate (BC), etc. Among these compounds, although PC having a relatively low freezing point of −49° C. is preferably used to exhibit favorable low-temperature performance, this compound may vigorously react with an anode of a battery during charge when a graphitized carbon with a high capacity is used as the anode. Therefore, instead of PC, which is difficult to use in large amounts, EC capable of forming a stable protection film at the anode is generally used. However, since EC is not entirely non-active with the cathode (or may also be slightly active with the cathode), decomposition of the electrolyte occurring at the anode and the cathode during charge/discharge of a battery may be a cause for deterioration in lifespan of the battery and EC has increased activity at a high temperature, thus being undesirable.
Accordingly, in order to solve the above problems and enhance lifespan of a battery at room temperature and high temperatures, Japanese Laid-Open Application No. 2000-123867 discloses a battery fabricated by adding a small amount of an ester compound, which has a cyclic molecular structure and C═C unsaturated bonds in a ring (such as vinylene carbonate), to an electrolyte. It is considered that such ester compound as an additive is decomposed at either an anode or a cathode and forms a film on a surface of the electrode so as to inhibit decomposition of the electrolyte. However, the above additive cannot completely prevent decomposition of the electrolyte.
Further, Japanese Laid-Open Application No. 2002-25611 discloses a battery fabricated by adding ethylene sulfite and vinylene carbonate to an electrolyte, while Japanese Laid-Open Application No. 2002-270230 proposes another battery fabricated by adding at least one of various ethylene sulfite compounds to an electrolyte. However, it was found that no additive described in these prior arts can exhibit desirable effects.
Accordingly, in consideration of performance at high temperatures, there is still a strong need for development of effective additives in secondary battery applications.